Enhanced uplink has been introduced as part of the release 6 of the third generation partnership project (3GPP) standards. The enhanced uplink operates on a rate request and grant mechanism. A wireless transmit/receive unit (WTRU) sends a rate request indicating the requested capacity, while a network responds with a rate grant to the rate request. The rate grant is generated by a Node B scheduler. The WTRU and a Node B use a hybrid automatic repeat request (HARQ) mechanism for transmissions over an enhanced dedicated channel (E-DCH).
For enhanced uplink transmission, two uplink physical channels, (E-DCH dedicated physical control channel (E-DPCCH) and an E-DCH dedicated physical data channel (E-DPDCH)), and three downlink physical channels, (E-DCH absolute grant channel (E-AGCH), E-DCH relative grant channel (E-RGCH), and E-DCH HARQ indicator channel (E-HICH)), have been introduced. The Node B may issue both absolute grants and relative grants. Rate grants are signaled in terms of a power ratio. Each WTRU maintains a serving grant that can be converted to a payload size.
WTRUs that make E-DCH transmissions have an E-DCH active set. The E-DCH active set includes all cells for which the WTRU has an established E-DCH radio link. The E-DCH active set is a subset of a dedicated channel (DCH) active set. A distinction is made between those radio links that are part of the E-DCH radio link set (RLS) and those that are not. The former includes radio links that share the same Node B as a serving Node B. Cells for non-serving radio links may only send relative grants in an effort to limit or control the uplink interference.
As part of ongoing evolution of the wideband code division multiple access (WCDMA) standard in 3GPP Release 8, a new work item has been established to incorporate E-DCH concepts for WTRUs in a CELL_FACH state. In Release 7 and earlier, the only uplink mechanism for WTRUs in a CELL_FACH state was a random access channel (RACH). The RACH is based on a slotted-Aloha mechanism with an acquisition indication. Before sending a message on a RACH, a WTRU tries to acquire the channel by sending a short preamble (made up of a randomly selected signature sequence in a randomly selected access slot). The WTRU then listens and waits for an acquisition indication from the universal terrestrial radio access network (UTRAN). If no indication is received, the WTRU ramps up its power and tries again (sending a randomly selected signature sequence in a randomly selected access slot). If an acquisition indication is received, the WTRU has effectively acquired the channel, and may transmit a RACH message part of finite duration. The initial preamble transmit power is established based on an open loop power control, whereas the ramp-up mechanism is used to further fine-tune the transmit power. The RACH message is transmitted at a fixed power offset from the last preamble and is of fixed size. Macro-diversity is not employed and the WTRU has no concept of active set for the RACH.
The new work item attempts to increase the uplink user plane and control plane throughput by assigning dedicated E-DCH resources after the initial WTRU power ramp up, (it is referred to “enhanced Uplink in CELL_FACH state and Idle Mode” or “enhanced RACH”). A WTRU transmits a RACH preamble in order to acquire a channel implementing power ramp-up. Once the RACH preamble is detected, a Node B transmits an acquisition indication (AI). After receiving the AI, the WTRU is assigned with an E-DCH resource for a subsequent E-RACH message transmission. The E-DCH resource assignment may be made either with the AI or with an enhanced set of AIs. The WTRU then transmits an E-RACH message and enters a contention resolution phase. The contention resolution phase is provided to solve potential collision of the E-RACH message. After transmission of all the data in the buffer, explicit indication from UTRAN, radio link failure, post verification failure, or expiry of a timer, the E-DCH resource is released.
A WTRU in a CELL_FACH state may use high speed downlink packet access (HSDPA) in the downlink.
However, this approach currently suffers from several problems. First, the initial transmissions on the high speed downlink channel may not be privy to channel quality information. In 3GPP Release 7, this was partially addressed by having the Node B use the channel quality information carried in an information element (IE), “Measured Results on RACH”. This IE is included in a number of layer 3 radio resource control (RRC) messages. In addition, a WTRU in a CELL_PCH state receiving dedicated control or data traffic is triggered to send channel quality information through a layer 3 measurement report upon reception of high speed downlink control traffic, (i.e., high speed shared control channel (HS-SCCH) with the WTRU address). However, as the feedback is sent through RRC signaling, it may be too slow for efficient modulation and coding control of the initial high speed downlink transmission.
Second, the 3GPP Release 7 approach is geared more toward WTRU-initiated control traffic, (for instance a CELL UPDATE). In a typical scenario, the WTRU would tag along channel quality information to the uplink RRC message. The network would then use this information to determine the allowed modulation and transport block size, and then send an RRC network response using the selected parameters. However, there may be some inefficiency if the uplink traffic is user-plane data traffic and does not carry any channel quality information, or is an RRC message that does not carry the IE: “Measured Results on RACH”, or if user-plane and control-plane traffic is network-initiated.
In both cases, the network may not have timely channel quality information and it would have to rely on the information received in the last IE: “Measured Results on RACH”. This inefficiency is likely to be more prevalent with enhanced RACH, as the network may decide to keep more WTRUs in a CELL_FACH state, for example to deal with asymmetric type applications, such as web browsing. It is likely that these WTRUs are kept in a CELL_FACH state, but that their enhanced RACH resources are released (for instance, after the WTRU has finished its transmission). As a result, any subsequent network-initiated downlink transmissions will not have “up-to-date” channel quality information. This would result in some inefficiency as the network would not be able to maximize the downlink transmission rate. Thus high speed downlink packet access (HSDPA) in a CELL_FACH state would benefit significantly from fast uplink feedback for both channel quality and HARQ feedback.